In conventional solar systems, using either flat plate liquid, concentrating liquid, or flat plate air solar collectors, an electronic differential sensing system, which senses the difference in temperature between storage and outlet temperatures, is normally used to turn on the system pump or blower. The normal differential temperature sensed lies in a range of between about 12.degree.-20.degree. F. Once the temperature differential is exceeded, the system pump will cause flow of liquid or air through the collector. The flowing fluid, or air, is then heated by the sun, and its energy is delivered to storage or is used directly.
In most present systems, the fluid flow is constant and is adjusted by means of a control valve which must be set for each installation.
Constant fluid flow through the collector is, however, not completely efficient. Optimally, one wishes to have a greater fluid flow rate through the collector when the collector panel temperature and/or the ambient air is hot; conversely, one wishes to have a lower fluid flow rate through the collector when the collector panel temperature and/or ambient air is cool. In this way, the greatest efficiency for the collector can be realized.
In a limited number of systems variable flow is achieved by varying the speed of the pump device using a proportional differential electronic control unit. However, variable speed pumps are extremely costly and render the use of such pumps economically impractical for many, smaller installations, such as may be used for homes and small office or apartment buildings.
To the best of my knowledge, no low-cost solar collectors are being made, or marketted, which rely on variable fluid flow through the solar collector, to increase efficiency, without requiring the use of variable speed pumps, as other costly electronic controls and sensing systems.